Linear motors are known to actuate elevators doors. For instance, U.S. Pat. No. 2,303,263 and U.S. Pat. No. 2,365,632, both to Fisher, each show a linear motor which opens and closes elevator doors.
Each linear motor comprises: a rectilinear stator having equal length magnetic portions alternating with non-magnetic portions; several electromagnets spaced apart and disposed and moveable longitudinally along the stator, the length of each electromagnet being equal to the length of one of the portions, and the distance between two successive electromagnets being one and two-thirds thirds times the length of an electromagnet; and, a means responsive to the relative movement of the electromagnets for energizing the electromagnets successively in sequence so that the movement occurs in one direction, or in reverse sequence so that the movement occurs in an opposite direction.
The motors have the following drawbacks: the relay system for providing power to the electromagnets is complicated, cumbersome and heavy; the motor operates at low efficiency because each electromagnet is powered, and is therefore driving, for a third of the time, whereas the other two-thirds of the time, the electromagnet is passive, the result of which is that only the electromagnet supplied with power is working and must pull the other two electromagnets; and at least three electromagnets must be used so as to obtain smooth driving of the doors, which leads to a relatively heavy and bulky motor.